Bash Functions: How to Write and Use Them (with Examples)

Applies to: Bash 5 on Linux and macOS (examples tested with Bash 5.1)

A Bash function is a named block of commands you can call like a small command. Functions make scripts shorter, easier to read and easier to change. Every example here was run in Bash 5.1.

Define and call a function

greet() {
    local name="${1:-world}"
    echo "Hello, $name"
}

greet            # Hello, world
greet Ucartz     # Hello, Ucartz

Define a function before you call it. The keyword form function greet { ... } also works in Bash, but greet() { ... } is more portable.

Arguments

• $1, $2...: the first, second argument.
• $#: the number of arguments.
• "$@": all arguments, each kept separate (use this to pass them on).

count_args() { echo "got $# arguments: $*"; }
count_args a "b c" d      # got 3 arguments: a b c d

Return values

return only sets an exit status (0 = success, 1 to 255 = failure). To give back data, print it and capture it:

add() { echo $(( $1 + $2 )); }
sum=$(add 2 3)
echo "sum=$sum"          # sum=5

backup_file() {
    local src="$1"
    [ -f "$src" ] || { echo "no such file: $src" >&2; return 1; }
    cp -- "$src" "$src.bak" && echo "saved $src.bak"
}
backup_file /nonexistent; echo "exit status: $?"   # exit status: 1

Send error messages to standard error (>&2), so they don’t end up in a variable that captures the output.

Local and global variables

Variables inside a function are global unless you mark them local:

x=outer
change() { local x=inner; y=global; }
change
echo "x=$x y=$y"       # x=outer y=global

Use local for everything a function uses internally, so it can’t overwrite variables elsewhere in the script.

List and reuse functions

declare -F                  # names of defined functions
declare -f greet            # show a function's code
source ~/lib/functions.sh   # load functions from a file

Put functions you use often in a file and source it from your scripts or from ~/.bashrc (Bash startup files).

Tips

• Quote variables ("$1") so that names with spaces don’t break.
• Start scripts with set -euo pipefail to stop on errors, unset variables and failed pipes.
• Check scripts with shellcheck (available in most distributions’ repositories).